1. Field of the Invention
The present invention relates to a magnetic recording medium, in particular a multilayer magnetic recording medium having a thin upper magnetic recording layer which has good electromagnetic recording properties, a smooth surface and improved lubricant absorption in the lower layer, and a process for its production.
2. Discussion of Background Information
Magnetic recording media, for example magnetic tapes, floppy disks or magnetic cards, have long been known for the recording of analog or digital audio and video signals or for data recording.
There has for some years been an increasing need for media which are suitable for increasingly high recording densities, i.e., capable of storing large quantities of information in a very small space. Increasingly small thicknesses of the recording layer and magnetic pigments having increasingly small dimensions and increasingly high coercive forces are a precondition for this.
Binder-containing double-layer magnetic media in which production difficulties, arising from the reduction in the thickness of the upper recording layer, and mechanical stability problems can be compensated by a relatively thick lower layer without having to dispense with a high recording density became established some time ago. They have higher stability and better productivity during manufacture compared with the magnetic recording media of the ME (metal evaporated) type, which are also known and are very suitable for high recording densities.
For information storage, they have a very smooth magnetic upper layer which is generally less than 0.5 μm thick and contains magnetic pigments having a high coercive force, preferably metal pigments or pigments comprising metal alloys. A thicker, likewise pigmented, binder-containing lower layer is present between the generally nonmagnetic substrate and the magnetic recording layer. On the one hand, the lower layer is indispensable for the production process of such double layers, particularly when these have to be applied wet-in-wet for obtaining a very thin magnetic recording layer. On the other hand, it also influences many other properties, in particular the tape smoothness and the mechanical properties of such magnetic media.
The lower layers used here are layers which contain both magnetic and nonmagnetic inorganic pigments, as well as layers which contain only nonmagnetic pigments in addition to binders and conventional additives. In publications and practical examples, it has now been shown that magnetic recording media having such a magnetic double layer and those having a nonmagnetic layer between substrate and recording layer substantially surpass the storability of corresponding magnetic particulate monolayer tapes having ferromagnetic metal powder in the recording layer.
It is known that, in the case of magnetic recording media having a double layer, the magnetic storability, i.e., the recording density, or the signal level depends primarily on the properties of the upper layer. Parameters which have an effect include their magnetic properties, their surface smoothness, the pigment type used and the thickness of the upper layer.
In addition, however, it has also been found that the lower layer, which does not come into contact with the head, also has considerable influence on the properties of the medium as a whole.
For example, it is only through specially established Theological properties of the lower layer dispersion in relation to the upper layer dispersion that it has become at all possible to achieve the desired small thickness of the upper layer in wet-in-wet coating. If, moreover, the lower layer has an excellent dispersion quality, it can cover roughnesses of the substrate film and thus form a good base layer for a smooth upper layer. On the other hand, with its high porosity and adapted hardness, it substantially supports the surface smoothing process in the calendering of the medium.
Further important functions of the lower layer are, for example, the establishment of electrical and mechanical properties of the tape, such as conductivity, scratch resistance, rigidity (modulus of elasticity) and good running properties (coefficients of friction). In order to maintain low coefficients of friction of the tape over long operating periods, the lower layer must be capable of storing a sufficiently large stock of lubricant, which subsequently gradually reaches the tape surface through diffusion.
In addition to the development of magnetic pigments having a high coercive force and recording layers having the desired properties, it has therefore long been the object of many development efforts also to provide suitable or improved lower layers in order to further to enhance the recording properties of magnetic, particulate double-layer recording media. In this way, it is also intended to increase their attractiveness compared with the abovementioned ME media.
It has proven advantageous to vary specific parameters in order to achieve an improvement in the lower layers thereby. These include, for example, the choice of particularly suitable lower layer pigments or pigment compositions, such as TiO2, carbon blacks, metal oxides, supporting pigments; the optimization of their shape and size, such as needles, spindles, finely divided character, homogeneity; and the improvement of their dispersibility, such as surface finishing, production process with little sintering. In addition to nonmagnetic pigments, magnetic pigments were also used in the lower layer, generally as a mixture with larger or smaller amounts of nonmagnetic pigments, with the result that desired additional properties could also be obtained in many cases.
Thus, DE-A-198 38 799 describes a double-layer magnetic recording medium which contains a magnetic pigment in the lower layer, which has a lower coercive force than the upper layer, with the result that particularly good overwritability of such double layers could be achieved.
EP-A-0 818 040 discloses a tape-like magnetic recording medium which has a plurality of layers and, owing to the use of weakly magnetic, acicular pigments in the lower layer, achieves, inter alia, excellent moduli of elasticity of double-layer tapes.
Such weakly magnetic pigments in the lower layer can, as described in DE-A-197 47 068, also lead to particularly good orientability of lamellar double-layer media.
Magnetic recording media having a plurality of layers, in which nonmagnetic pigments alone, as a mixture with magnetic pigments or as a mixture with magnetically soft pigments are used in the lower layer, have also been disclosed. Such media have been described, for example, in EP-A-0 592 922 and in U.S. Pat. No. 5,458,948.
In spite of a different composition of the lower layers, the media exhibit no significant differences in their electromagnetic and other properties.
U.S. Pat. No. 5,792,570 relates, inter alia, to a multilayer recording medium in which the smoothness of the magnetic layer and the C/N ratio in the radio frequency range are said to be improved by the use of magnetically soft pigments in the magnetic recording layer and, if required, also in a lower layer. However, nonmagnetic lower layers and lower layers comprising magnetically soft pigments show no differences in the desired properties.
U.S. Pat. No. 5,705,268 describes multilayer magnetic recording media which have a magnetic recording layer comprising ferromagnetic metal powders or hexagonal ferrites and whose lower layer contains a magnetically soft pigment having a narrow particle size distribution. These recording media are said to have a smoother magnetic layer surface, better overwritability and a better output level in the radio frequency range compared with media having nonmagnetic lower layers.
The magnetically soft pigments used in the lower layer are either ferrites of the spinel type or magnetically soft metal alloy powders, which can generally be prepared only by expensive processes and/or by the use of relatively high temperatures.
In these media, however, the porosity of the lower layer is as a rule not sufficient for absorbing relatively large amounts of lubricants, which is evident from higher coefficients of friction of the tape after prolonged operation. Furthermore, there is still a need for magnetic recording media which have even better surface smoothness and improved output levels at short recording wavelengths.